Fiber amplifier having efficient pump power utilization

Abstract: The present invention relates to a multi-stage fiber amplifier in which first and second stages are connected by means including a filter for attenuating the amplified spontaneous emission. The pump source consists of a pair of light sources that are connected to a coupler that splits the power equally to two output legs. All of the pump power from the second coupler output leg is supplied to the second stage. A portion of the pump power from the second coupler output leg is converted in the first stage to amplified signal and amplified spontaneous emission, and the remainder of the power from that leg is supplied to the second stage.

Description: Background of the Invention The present invention relates to an optical amplifier for optical communications, and more particularly, to a multi-stage fiber amplifier in which the total available pump power, which is supplied from a single source, is optimally divided between the stages to provide maximum gain and minimum noise for a given spectral response. High output power amplifiers are required for many applications, including, for example, multi-wavelength systems. One of the key performance characteristics of an amplifier is its output power, which is largely governed by how efficiently the pump light is converted into signal light. In amplifiers doped, for example with erbium, but also in amplifiers doped with other gain ions, the input available for amplification comprises not only the signal but also the spontaneous emission (SE). To reduce amplified spontaneous emission (ASE), pump power cannot be reduced, as it is needed for conversion to signal power. (Fig. 1). Previous proposals to reduce ASE amplification, require additional circuitry and components, such as filters, multiplexer couplers, or several pump sources (see IEEE Photonics Technology Letters, Vol. 4, page 1340-1348, December 1992; Electronic Letters, Vol. 26 page 661, 662, May 10, 1990; U.S. Patent 5,050,949. GB-A-2 253 514 discloses a fiber amplifier according to the preamble of claims 1 and 9. The invention also applies to fiber amplifiers containing gain ions other than erbium, since ASE also diverts pump power from the signal in amplifiers utilizing gain ions other than erbium. Summary of the Invention The invention is defined in claims 1 and 9. It is therefore an object of the present invention to provide a multi-stage fiber amplifier in which pump power from a single source is unequally coupled to the two stages in such a manner that output power is optimized for a given spectral response. Another object is to provide a fiber amplifier that overcomes the disadvantages of prior art devices. Briefly, the present invention relates to a fiber amplifier comprising first and second fiber amplifier stages, the output spectrum from the first stage including a first band of wavelengths that is primarily attributable to ASE. The first and second stages are connected by means including a filter for attenuating the first band of wavelengths. Distribution means connects pump light from a source to the first and second stages such that less than half of the pump power is converted to signal and amplified spontaneous emission in the first amplifier stage. In one embodiment the pump source consists of a pair of light sources connected to a coupler that splits the power equally to two output legs. All of the pump power from the second coupler output leg is supplied to the second stage. A portion of the pump power from the first coupler output leg is converted in the first stage to amplified signal and ASE, and the remainder of the power from that leg is supplied to the second stage. Brief Description of the Drawings Fig. 1 is a schematic illustration of a gain fiber. Fig. 2 is a graph showing the gain spectrum of an erbium-aluminum-doped germania silicate fiber amplifier. Fig. 3 is a schematic illustration of a fiber amplifier in accordance with the present invention. Fig. 4 is a schematic illustration showing division of pump power. Figs. 5, 6 and 7 are schematic illustrations of embodiments of the invention. Description of the Preferred Embodiments To maximize the pump-to-signal conversion efficiency, the amount of light converted to ASE should be minimized. In accordance with this invention, pump power from a single source is divided such that pump power consumption is reduced in the primary stage of the amplifier, thus reducing the power converted to ASE. Excess pump power is directed to a later stage in the amplifier where the signal will be larger and better conversion efficiency can be attained. A basic schematic diagram of the invention is shown in Fig. 3. The amplifier comprises a primary stage 31 which is connected to a secondary stage 32 by an ASE filter 33. ASE filter 33, as well as the ASE filters of later described amplifiers, can be one that diverts light between 1530-1540 nm to an output telemetry port. It is in this spectral region that the peak of ASE emission is found for a fully inverted amplifier. A given pump budget is available from pump source 34. A given amount x of the available pump power is supplied to stage 31, wherein x is less than 50%. The remaining (1-x) of the available power is supplied to stage 32. The value of x is typically between about 10% and 49%. At values below 10%, the noise performance is seriously degraded. There is obviously not much improvement in efficiency of pump-to-signal conversion when x is 49%. The value of x is thus often selected to be between about 20% and 30%. To determine the value of x for a given amplifier, the amount of amplifier noise that can be tolerated in the system is initially determined. The gain is then optimized for that noise performance by appropriately apportioning the pump power budget to the first and second amplifier stages. As shown in Fig. 4, the pump source may be a laser diode 37 connected to stages 1 and 2 by a coupler 38 that splits the pump power such that, at the pump wavelength, less than half (e.g. 25%) of the power is supplied to stage 1, The remainder of the pump power being directed to the second stage. This configuration requires few components, but it does not provide the amplifier with the soft fail function which will be described in the remaining embodiments. Fig. 5 illustrates a multi-stage fiber amplifier that is pumped by a single source which divides the pump power between the stages so that the amplifier achieves efficient pump power-to-signal conversion efficiency. Laser diodes 41 and 42 of source 40 are connected to a 3 dB coupler 43 which provides equal amounts of pump power to 3 dB coupler 44 and wavelength division multiplexer (WDM) coupler 45. The gain of the amplifier will drop by no more than 3 dB if one of the sources fails, due to the use of coupler 43 in the pump source. Without this soft fail architecture, the gain will drop by more than 3 dB when one of the pump diodes fails. Coupler 44 provides equal amounts of pump power to WDM couplers 46 and 47. The signal of wavelength λ s , which is applied to coupler 46, is amplified by first fiber amplifier stage 48. The amplified signal is connected to gain fiber 54 by pump filter 50, ASE filter 51, isolator 52 and WDM coupler 47. Pump filter 50 is employed when ASE filter 51 is of the type whose performance is degraded by pump light. The amplified signal from gain fiber 54 is connected to gain fiber 56 by WDM coupler 45. The pump-to-signal conversion efficiency is improved in this embodiment by employing only 25% of the pump power in the first stage 48, and supplying 75% of the pump power to the stage including gain fibers 54 and 56, which is located after ASE filter 51. Isolators 49, 52 and 53 suppress reflection noise. An optional pump filter 55 protects downstream elements from pump light that might be deleterious thereto. Comparison Amplifier 1 was constructed to evaluate the performance of the amplifier of Fig. 5. Comparison Amplifier 1 was similar to the amplifier of Fig. 5 except that the second stage consisted of a single gain fiber and the pump power was supplied to the two stages directly from source 40, i.e. each stage received 50% of the available power. The output power of the amplifier constructed in accordance with Fig. 5 was more than 1.5 dB larger than the output power of Comparison Amplifier 1. An accurate numerical model was constructed for the amplifier of Fig. 5 and for Comparison Amplifier 1. It showed that the output power of an amplifier constructed in accordance with Fig. 5 would have 1.4 dB greater output power than Comparative Amplifier 1 at equivalent noise and gain spectrum. In Fig. 6 laser diodes 61 and 62 are connected to a 3 dB coupler 63 which provides equal amounts of pump power to WDM couplers 64 and 65. The signal of wavelength λ s , which is applied to coupler 64, is amplified by a first fiber amplifier stage comprising gain fiber 67. The amplified signal is connected to gain fiber 68 by WDM coupler 70, ASE filter 71, isolator 72 and WDM coupler 74. The length of first stage gain fiber 67 is insufficient to convert all of the pump power supplied thereto by couplers 63 and 64 to signal and ASE. Gain fiber 67 might absorb only 50% of the pump power, for example. The remnant pump power from gain fiber 67 is connected to gain fiber 68 through couplers 70 and 74 thus pumping gain fiber 68 in the forward direction. Gain fiber 68 is also pumped in the reverse direction by the pump power supplied to it from coupler 65. The amplified signal is coupled from gain fiber 68 to output fiber 75 by WDM coupler 65 and isolator 76. If, as suggested above, gain fiber 67 absorbs only 50% of the pump power supplied to it, then gain fiber 67 utilizes essentially 25% of the available pump power budget from source 60, and gain fiber 68 utilizes essentially 75% of the available pump power budget from source 60. As discussed above, this division of pump power improves the pump-to-signal conversion efficiency. Due to the finite signal crosstalk of WDM couplers 70 and 74, some signal light leaks into the pump path a. This signal will be out of synchronization with the principle signal from path b where the paths rejoin at coupler 74 due to inevitable small differences in path length between the two paths. This unsynchronized signal will be a source of added noise. It has been calculated that with WDM couplers 70 and 74 having 15 dB crosstalk at the signal wavelength, this 30 dB attenuated signal will degrade the noise figure by as much as 26 dB, when the path lengths differ by more than the source coherence length. To block this secondary signal path, which would ordinarily occur through pump path a, a signal attenuating fiber 73 is added to path a. It has been calculated that 60 dB signal attenuation in path a would eliminate this additional source of noise. If the gain fibers were erbium-doped, the filtering function could easily be obtained with one meter of praseodymium-doped fiber, for example. Fig. 7 shows an alternate design for the second stage 66 of Fig. 6. Gain fiber 68 is replaced by two gain fibers 81 and 82. Connecting fibers a, b and c, which enter second stage 66 of Fig. 6, are also shown in Fig. 7. Gain fiber 81 is pumped in the forward direction by pump power supplied thereto via connecting fiber a and WDM coupler 74'. The pump power from connecting fiber c and the amplified signal from gain fiber 81 are coupled to gain fiber 82 by WDM coupler 84, whereby gain fiber 82 is also forward pumped. The amplified output is connected to output fiber 86 by pump filter 87 and isolator 88. An accurate numerical model was also constructed for the amplifier of Fig. 7. It showed that the output power of an amplifier constructed as in Fig. 7 would have 1.9 dB greater output power than Comparative Amplifier 1 at equivalent noise and gain spectrum.

Claims: A fiber amplifier comprising a first fiber amplifier stage (31, 48, 67), the output spectrum from said first stage including a first band of wavelengths that is primarily attributable to amplified spontaneous emission, a second fiber amplifier stage (32, 54, 56; 68; 81, 82), connecting means for connecting said first stage to said second stage, a source (34, 37, 40, 60) of pump light, distribution means (38; 43, 44, 45, 46, 47; 63, 64, 65, 70, 73, 74; 74', 84) for connecting pump light from said source to said first stage and said second stage such that less than half of said pump power is converted to signal and amplified spontaneous emission in said first amplifier stage, characterized in that said distribution means are such that said first stage is forward pumped and said connecting means include a filter (33, 51, 71) for attenuating said first band of wavelengths. A fiber amplifier in accordance with claim 1 wherein said distribution means includes a wavelength division multiplexer coupler (46, 64) for connecting pump light from said source (40, 60) to said first stage and for connecting an input signal to said first stage. A fiber amplifier in accordance with claim 1 or 2 wherein said source of pump light comprises a single light emitting diode (37) and said distribution means comprises coupler means (38) that unequally splits the pump light from said diode to first and second output legs, said first output leg being connected to said first stage and said second output leg being connected to said second stage. A fiber amplifier in accordance with claim 1 or 2 wherein said source comprises a first coupler (43) having two input legs and two output legs, the characteristics of said first coupler being such that substantially one-half the power supplied to each of said first coupler input legs is coupled to each of said first coupler output legs, and first and second sources (41, 42) of pump light respectively connected to said first and second input legs, and said distribution means comprises means (45) for connecting all of the pump light from the second output leg of said first coupler (43) to said second stage, means (44, 46) for connecting a portion of the pump light from the first output leg of said first coupler to said first stage, and means (44, 47) for connecting the remainder of the pump light from the first output leg of said first coupler to said second stage. A fiber amplifier in accordance with claim 4 wherein said distribution means comprise means (45) for connecting the second output leg of said first coupler to said second stage, a second coupler (44) having an input leg and two output legs, the characteristics of said second coupler being such that a portion of the power supplied to said second coupler input leg is coupled to the first output leg of said second coupler, the remainder of the power supplied to said second coupler input leg being coupled to the second output leg of said second coupler, said second coupler input leg being connected to the first output leg of said first coupler, means (46) for connecting the first output leg of said second coupler to said first stage, and means (47) for connecting the second output leg of said second coupler to said second stage. A fiber amplifier in accordance with claim 1 or 2 wherein said first stage comprises a gain fiber (67), the length of which is insufficient to convert all the pump power supplied thereto to amplified signal and amplified spontaneous emission, (a) said distribution means including means (70, 74) for connecting pump light from the output of said first stage to said second stage and/or (b) said connecting means including a wavelength division multiplexer coupler (70), one output of which is connected to said amplified spontaneous emission filter, the other output of which is connected to said second stage. A fiber amplifier in accordance with claim I or 2 wherein said pump light source comprises at least one laser diode and a coupler (63) for apportioning the power from said diode to first and second output legs, said second stage comprises a single gain fiber (68) having first and second ends; said second coupler output leg being connected to the second end of said single gain fiber; said first stage comprising a first gain fiber (67), the length of which is insufficient to convert pump power supplied thereto to signal and amplified spontaneous emission; said connecting means including a wavelength division multiplexer coupler (70), one output [leg] of which is connected to said amplified spontaneous emission filter, the other output [leg] of said wavelength division multiplexer coupler being connected to the first end of said single gain fiber. A fiber amplifier in accordance with claim 1 or 2 wherein said pump light source comprises at least one laser diode and a coupler (63) for apportioning the power from said diode to first and second output legs, said second stage comprises the serial connection of second and third gain fibers (81, 82); said second coupler output leg being connected to said third gain fiber (82); the length of said first stage being insufficient to convert all the pump power supplied thereto to amplified signal and amplified spontaneous emission; said connecting means includes a wavelength division multiplexer coupler, one output of which is connected to said amplified spontaneous emission filter, other output of which is connected to said second gain fiber. A fiber amplifier comprising a first fiber amplifier stage (48, 67), a second fiber amplifier stage (54, 56; 68; 81, 82), connecting means for connecting said first stage to said second stage, a source (40, 60) of pump light having a first and a second output leg distribution means (45; 65; 84) for connecting all of the light from said second source output leg to said second stage, characterised in that equal amounts of pump power appear in both said output legs and in that said fiber amplifier also comprises a wavelength division multiplexer coupler (46, 64) for connecting an input signal to said first stage, filter means (51, 57) in said connecting means for attenuating amplified spontaneous emission appearing at the output of said first amplifier stage, distribution means (44) for connecting a portion of the light from said first source output leg to said wavelength division multiplexer coupler (46, 64) whereby said first fiber amplifier stage (48, 67) is forward pumped, and distribution means (44, 47 ; 64, 67, 70, 73, 74, 74') for connecting the remainder of the light from said first source output leg to said second stage.

Inventors: BUTTON LESLIE JAMES, NEWHOUSE MARK ANDREW, BUTTON, LESLIE JAMES, NEWHOUSE, MARK ANDREW
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 1994
Main classification: H01S3
IPC codes: H04B10, H01S3, G02F1